Method of making mositureesistant paper



Patented Jan. 23, 1,951 I METHOD OF MAKING MOISTURE- RESISTANT PAPER Chester H. Child, Methuen, Mass.

No Drawing. Application November 29, 1944, Serial No. 565,791

Claims.

. This invention is concerned with paper making and more specificallywith the fabrication of a flexible moisture-resistant paper particularly adapted for use as insulating tape in the manufacture of non-metallic sheath cable, although it has characteristics adapting it for widespread other uses, for instance, as backings for finishing papers in the sandpaper art.

The requirement that insulation be kept as substantially moisture free as possible has been a great hindrance to the use of paper-naturally moisture absorbent-as an insulating medium. Steps toward rendering the paper moisture-resistant have been taken by immersing rolls of paper insulating tape in hot paraflin wax. This treatment provides an acceptable moisture-resistant surface on the paper but the fact remains that the treatment is essentially a surface treatment only. There is nouniform impregnation of the sheet and the internal moisture-resistance of tapes'so treated is far from acceptable. The external wax coating is often disrupted during winding of the tape about a cable by reason of the tension, distortion, or for other reasons, leaving insufliciently protected points on the surface of the sheet through which moisture gains access to the relatively unimpregnated internal body of the sheet. Further, whenever the tape is out after dipping, a raw unimpregnated internal body edge is laid open K and will attract and absorb moisture. This difficulty is especially encountered at cable ends where exposed internal body edges pick up moisture which is spread along the cable by capillary action in the internal body of the paperinsulation.

It is a main object of this invention to provide a sheet of ordinary paper of the wrapping paper type, 'as of unbleached sulfate fiber, which exhibits a uniform lack of moisture absorption throughout, and wherein surface disruptions and cut edges do not affect the moisture-resistant characteristics of the sheet.

For the purposes of this application, a paper of suitable moisture resistance is one which, when cut in a 1 inch by 8 inch strip with open raw edges and completely submerged in tap water at 70 F. for a period of one hour, will gain in weight in an amount less than 60% of its air dry weight prior to submersion. Papers manufactured in accordance with this invention in commercial paper making operations have attained va ues averaging, in the above tests, gains of only about 43%. These percentages are to be contrasted with present-day waxdipped papers which exhibit gains averaging in excess of about in the above test.

The moisture resistance of papers of this invention is obtained by incorporating in the paper making stock, prior to formation of a web or sheet therefrom, particular moisture-resistance imparting materials, as distinguished from treating a formed paper sheet with these or other materials. By such incorporation, a sheet of far superior external-internal uniformity can be and is secured.

Addition of moisture and water-resistance imparting ingredients to paper making stock in the beater has been common, but satisfactory moisture resistance of the type heretofore described has never been secured. For instance, while waxes have been incorporated into the beater, small amounts only, insufiicient to attain the moisture resistance characteristics attained in papers of this invention, have been'retained; and even then, the wax material tends to bloom to the surface during the drying operations concomitarit to usual paper making practice. Slippage difficulty in the paper making operations is also encountered. While somewhat better retention and less blooming is evidenced when the paper making stock is formed into hydrocellulose by prolonged hydration, such parchmentizing of the paper is expensive, involving beating times running as high as 48 hours, and imparts qualities of transparency and grease proof resistance which are needless so far as insulation is concerned. The papers of this invention require only ordinary beating times ranging from only two to four hours. Further, parchmentized papers do not have the flexibility of ordinary kraft papers and have slip qualities which render them exceedingly difficult to handle during cable winding. So far as applicant is aware, parchmentized papers have not been found acceptable in the cable insulation industry.

The paper with which I am particularly concerned is therefore a flexible, relatively porous, usually opaque, unbleached or semi-bleached sulfate kraft paper having the general appearance of ordinary kraft Wrapping paper. Its porosity may be in the range of 35 to 40 seconds per. cc. (Gurley densometer), as distinguished from the entirely different range of porosity of parchmentized papers running from 1500 to 2000 seconds per 100 cc.

In accordance with this invention, a wax is incorporated into a rosin-sized paper making stock in quantities greater than that heretofore t g t capable of being retained in the finished sheet, is precipitated onto the fibers of the stock, and is held thereon and bonded thereto by incorporation into the paper making stock of relatively small amounts of a water-soluble resin of the wet-strengthening type, i. e., one which is convertible to water-insolubility during or after formation of a sheet from the stock, thus providing a rosin-wax-resin impregnated paper of high internal moisture resistance. Such water-soluble curable resins are available on the market, the most common being urea-formaldehyde and melamine resins.

Production can be made in all weights of fourdrinier papers, for example 40 lb. to 180 lb. papers corresponding to caliper of .0038 inch to .018 inch, and in colored shades including black and white. The paper can be manufactured on ordinary paper making machinery without the use of any auxiliary equipment, and regular machine speeds of operation are utilized. The rolls of paper are formed on the regular winder and the particular freeness requirements of the type 4 water-soluble curable resin,. for. instance, of a urea-formaldehyde resin.

After thorough mixing of the urea-formald- During theseoperations, the paperis subject- I ed to the normal drier temperatures ranging from 200 F. to 220 R, which will result in an insolubilizing of theurea-formaldehyde resin in such manner that the resin, in conjunction with the rosin size, acts to keep the wax fully bound Preferably, and where high surface repellency including large surface contact angle is desirable, I utilize a high melting point (165 F.) microcrystalline parailin wax although papers having microcrystalline or amorphous paflin waxes of lower melting points (down to 122 F.) or petrolatum hydrocarbon waxes of the same or lower melting point (for instance 135 F. or down to 122 F.) incorporated therein in accordance with this invention have been found far superior to wax dipped papers.

Unbleached or semi-bleached sulfate wood pulp is added to the beater engine in accordance with ordinary paper making practice and upon complete vfiber disintegration a rosin size is added, in an amount from 1 to 5%, preferably 2.5%, on the weight of the dry fiber, while the stock has an adjusted pH in the range of 6.8 to 7.5. In order to attain this pH range, I recommend that white water not be used in the beater due to the acid condition created by the hydrolysis of alum accumulations.

A suitable rosin size is a plasticized paper size which comprises 80% by weight of rosin and 20% by weight of a 135 M. P. microcrystalline paraffin wax. However, other' preferably neutral rosin sizes may be utilized such as those 'to the fibers preventing blooming of the wax to the surface. Although the wax functions as a lubricant in the paper to prevent any tendency toward brittleness, slippage on the paper making rolls is not encountered, probably'due to the anti-slip qualities of both the rosin and the resin.

Where wet strength is of importance in addition to moisture resistance, the paper may be subjected to a flash heat such as 350 F. for a ten second contact. To secure best results the flash heat, as from infra red lamps, hot air or drier surface contact, is applied when the paper has less than 20% of moisture remaining.

In the above outline of procedure it will be noted that the amount of wax is designated as from 2 to and the amount of urea-formaldehyde resin is also from 2 to 5%. In general, I have found that best results are secured when the resin-to-wax ratio is about one to one, figured on the total wax content whether added as part of the rosin size or in the wax emulsion. Accordingly, preferred quantities have been indicated, in the case of awax-containing rosin size, as 2.5% of wax and 3% of resin since the amount of wax in the wax containing rosin size containing from 0 to by weight of amorphous paraflin wax of 122 to 124 M. P. or one having a high free rosin content.

About minutes before the end of the beating cycle, there is added to the beater from about 2 to 5% dry weight, preferably 2.5%, based on the dry weight of the fiber, of a microcrystalline paraffin wax of meltin point 165 in an acid unstable aqueous emulsion. I have found it convenient to utilize an emulsion of about 10% wax concentration secured by dilutin with water at 100 F.

After the desired freeness has been attained, usually about 20 minutes before the end of the beater cycle, the rosin and wax are precipitated on the fibers by acidifying the stock, for example by addition of suflicient paper maker's alum, for instance one containing approximately 17% alumina, or of other precipitating agent,

to lower the pH to from 4 to 4.4, preferably 4.2.

There is then added to the beater from 2 to 5% dry weight, preferably 3%, based on the dry weight of the fiber, of an aqueous solution of the increases the total amount of wax incorporated, bringing it up to a ratio of approximately 1 to 1' with the resin. In the case of the above mentioned rosin size, containing 20% of wax, 2.5% of the size, previously mentioned as a preferred amount, comprises 2% of actual rosin and 0.5% of wax. This amount of wax in the rosin size plus the 2.5% of wax additionally incorporated in the form of an aqueou wax emulsion provide a total of 3% of wax. While superior results are secured when microcrystalline paraflin wax of 165 M. P. is used, adequate results having a moisture resistance of not more than 60% gain are secured when amorphous or crystalline hydrocarbon waxes of melting point range from 165 down to 122 F., suitably emulsified or otherwise dispersed in aqueous solution, are substituted.

Where a melamine or other water-soluble wetstrengthening resin convertible to water-insolubility is utilized instead of urea-formaldehyde resin, the same proportions may be observed.

Addition of the rosin, wax and resin may be made at usual beater temperature, e. g. 90 F.

In the case of neutral water-soluble curable resins of the wet-strengthening type, for instance certain of the urea-formaldehyde resins, the resin may be added to the paper making stock prior to cycle and Just prior to acidifying the stock. The proportions may be similar to those observed in connection with the separate addition heretofore described. n the whole, however, end products produced with pre-precipitant addition of the resin have not been as uniform and homogeneous as those prepared by the preferred process hereinbefore described; spottinessz, which may not be of consequence in some papers, is likely to be encountered. Since melamine resins are acidic, ttihey are not adapted for pre-precipitant addi- Papers formed in accordance with this invention do not show deterioration upon storage and in fact the water-repellency and wet strength characteristics exhibit a tendency to improve upon aging. Likewise the paper is non-toxic, durable, and does not grow rancid in storage.

While I have described the invention with ref erence to its application to the production of unbleached or semi-bleached sulfate kraft flexible paper, it will be understood that if sufiicient modifications of the fiber treatment are made, in accordance with conventional paper making modifications, a more rigid or rigid sheet may be made for the purpose of producing a base stock for making cartons, fiber cans, and containers.

In general I have found that particular care should be taken in formation of the sheets to'prevent the presence of "voids" which might act as water pores. Too free sheets should therefore not be attempted.

Because of high wet tensile strength, papers of this invention are very suitable for use as wetpack wrapping, either in sheet or bag form.

paper making stock prior to the formation of a web therefrom, about 1 to 5% of rosin size based on the dry weight of the pulp and about 2 to 5% of parafiin wax based on the dry weight of the pulp, said wax being in the form of an acid unstable aqueous emulsion, precipitating the wax and rosin on to the fibers by acidifying the stock with alum subsequent to the incorporation of wax and rosin, bonding the precipitated wax to the fibers by means of water soluble, curable ureaformaldehyde resin in an amount comprising about 2 to 5% of the dry weight of the pulp, said resin being added to the acidified stock in the form of an aqueous solution, forming the stock into a web and curing the resin, said paper bein characterized by substantially reduced water absorbency, low slippage and high flexibility.

2. A method of making a moisture resistant paper which includes the steps of adding to the paper making stock prior to the formation of a web therefrom, about 1 to 5% of rosin size based on the dry weight of the pulp and about 2 to 5% of paraifin wax based on the dry weight of the pulp, said wax being in the form of an acid unstable aqueous emulsion, precipitating the wax and rosin on to the fibers by acidifying the stock with alum subsequent to the incorporation of said wax and rosin, bonding the precipitated wax to the fibers by means of water soluble, curable ureaformaldehyde resin in an amount comprising about 2 to 5% of the dry weight of the pulp, said resin being added to the acidified stock in the form of an aqueous solution, said wax and said resin being in a ratio of about 1 to 1, forming the stock into a web and curing the resin, said paper being characterized by substantially reduced water absorbency, low slippage and high flexibility.

3. A method of making a moisture resistant 8 paper which includes the steps of adding to the paper making stock prior to the formation of a web therefrom, about i to 5% of rosin size based on the dry weight of the pulp and about 2 to 5% of paraffin wax based on the dry weight of the pulp, said wax having a melting point of about 122 to F., and being in the form of an acid unstable aqueous emulsion, precipitating the wax' and rosin on to the fibers by acidifying the stock with alum subsequent to the incorporation of said wax and rosin, bonding the precipitated wax to the fibers by means of water soluble, curable ureaformaldehyde resin in an amount comprising about 2 to 5% of the dry weight of the pulp, said resin being added to the acidified stock in the form of an aqueous solution, forming the stock into a web and curing the resin, 'said paper being characterized by substantially reduced water absorbency, lowslippage and high flexibility. 4. The method of making a moisture resistant paper which includes the steps of adding o Paper making stock prior to the formation of a web therefrom about 1 to 5% rosin size based on the dry weight of the pulp and about 2 to' 5% of paraffin wax based on the dry weight of the pulp, said wax being in the form of an acid unstable aqueous emulsion, precipitating the wax and rosin on to the fibers by acidifying the stock with alum subsequent to the incorporation of said wax and rosin, bonding the precipitated wax to the fibers by means of a water soluble, curable thermosetting resin selected from the group consisting of ureaformaldehyde resin and melamine resin in an amount comprising about 2% to 5% of the dry weight of the pulp, said resin being added tothe acidified stock in the form of an aqueous solution, forming the stock into a web and ouring the resin, said paper being characterized by substantially reduced water absorbency, low slippage and high flexibility.

5. A method of making a moisture resistant paper which includes the steps of adding to the paper making stock prior to the formation of a web therefrom, about 2% of rosin based on the dry weight of the pulp and about 3% of paraflin wax based onthe dry weight of the pulp, said wax having a melting point of about 122 to 165 F. and being in the form of an acid unstable aqueous emulsion, precipitating the wax and rosin on to the fibers by acidifying the stock with alum subsequent to the incorporation of said wax and rosin, bonding the precipitated wax to the fibers by means of water soluble, curable ureaformaldehyde resin in an amount comprising about 3% of the dry weight of the pulp, said resin being added to the acidified stock in the form of an aqueous solution, forming the stock into a web and curing the resin, said paper being characterized by substantially reduced water absorbency, low slippage and high flexibility.

CHESTER H; CHILD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Num er Name Date 2,096,129 Neitzke Oct. 19, 1937 2,352,922 Thomas July 4, 1944 2,407,376 Maxwell Sept. 10, 1946 2,454,532 Walter Nov. 23, 1948 (Other references on following page) 7 8 FOREIGN PA'I'ENTS Chemistry 01 Pulp and Paper Making, Number Country Date Sutermelster (3rd edition, 1941), pages 306, 307,

5 1,865 Gre tBritain A 15,1939 ssgss re; Britain J u ly s, 1940 "Manufacture 0f P1111) and Paper, (3rd edition,

\ 5 1938), vol. 4, Sec. 5, pages 15, 16, 18, 24-26.

OTHER REFERENCES i Paper Industry and Paper World," July 1943, Timber Utilization," Paris, July 26-28, 1937, No.

page 426, Sept. 1944, pages 154 and 75a. 6. 17. Spe al op e -9 Y Paper Trade- Journal, Nov. 3. 1932, pages Res/1110115 p y 1944. P es 29-31, May 13, 1943, page 41, Dec.2, 1943, pages 10 Report of Third International Conference on 

1. A METHOD OF MAKING A MOSITURE RESISTANT PAPER WHICH INCLUDES THE STEPS OF ADDING TO THE PAPER MAKING STOCK PRIOR TO THE FORMATION OF A WEB THEREFROM, ABOUT 1 TO 5% OF ROSIN SIZE BASED ON THE DRY WEIGHT OF THE PULP AND ABOUT 2 TO 5% OF PARAFFIN WAX BASED ON THE DRY WEIGHT OF THE PULP, SAID WAX BEING IN THE FORM OF AN ACID UNSTABLE AQUEOUS EMULSION, PRECIPITATING THE WAX AND ROSIN ON THE FIBERS BY ACIDIFYING THE STOCK WITH ALUM SUBSEQUENT TO THE INCORPORATION OF WAX AND ROSIN, BONDING THE PRECIPITATED WAX TO THE FIBERS BY MEANS OF WATER SOLUBLE, CURABLE UREAFORMALDEHYDE RESIN IN AN AMOUNT COMPRISING ABOUT 2 TO 5% OF THE DRY WEIGHT OF THE PULP, SAID RESIN BEING ADDED TO THE ACIDIFIED STOCK IN THE FORM OF AN AQUEOUS SOLUTION, FORMING THE STOCK INTO A WEB AND CURING THE RESIN, SAID PAPER BEING CHARACTERIZED BY SUBSTANTIALLY REDUCED WATER ABSORBENCY, LOW SLIPPAGE AND HIGH FLEXIBILITY. 